JP4452992B2 - Manufacturing method of sintered body for solid electrolytic capacitor - Google Patents

Description

The present invention relates to a method for manufacturing a solid electrolytic capacitor. In particular, a method of manufacturing a burn sintered body for a solid electrolytic capacitor. Furthermore, the present invention relates to a solid electrolytic capacitor using the same.

Current electronic devices such as cellular phones and personal computers use small and large capacitors. Among various capacitors, a tantalum capacitor is a small-sized and large-capacity capacitor having a large capacity per unit volume, and has good thermal stability and electrical characteristics. For the anode body of this tantalum capacitor, a porous sintered body of fine tantalum powder is used in order to increase the surface area per unit volume.
However, tantalum ore is rare in the earth and the place where it is produced is limited. For this reason, the supply of tantalum tends to be unstable, and the price is likely to rise. In order to increase the capacity, it is necessary to further refine the powder to increase the surface area. However, if the powder is refined, the strength of the sintered body is reduced and it is difficult to ensure the pore diameter of the sintered body. When the pore diameter becomes small, impregnation with the cathode material becomes difficult, and a predetermined capacitor performance cannot be expressed. Therefore, there are proposals for capacitors using niobium pentoxide dielectric film, which has abundant reserve as resources more than 10 times that of tantalum and has a large relative dielectric constant as a dielectric diaphragm, and uses niobium metal powder and niobium oxide powder as anode materials. Has been made.

  However, the solid electrolytic capacitor having this configuration has a problem in the stability of the dielectric layer made of niobium oxide. That is, when niobium is anodized at a high voltage, an amorphous oxide film is crystallized to increase leakage current and increase the frequency of capacitor failure. In addition, since the dielectric film made of niobium oxide has many oxygen defects and is unstable to heat, the leakage current characteristics change or the capacitor capacity fluctuates due to the thermal history when reflow soldering the capacitor. There are problems such as. Furthermore, since the anode is a very porous sintered body, when its strength is weakened, the leakage current is increased.

  In order to solve these problems, a method of alloying niobium and aluminum or a method of alloying the niobium surface has been proposed. By doing so, since there is aluminum on the surface of the niobium metal, the formed dielectric film is a composite of niobium oxide and aluminum oxide, and this combined effect reduces oxygen defects in the film. It supplements and contributes to the stabilization of niobium oxide. In addition, if aluminum oxide is nano-dispersed near niobium oxide or a composite oxide of aluminum oxide and niobium oxide is formed, crystallization of niobium oxide is suppressed and the amorphous structure is stabilized. Therefore, the dielectric film exists stably up to a high voltage, and the withstand voltage is improved. Further, since the niobium-aluminum alloy is harder than pure niobium metal, the buckling strength of the anode porous sintered body is improved, so that stress due to the stress from the resin and heat load such as solder reflow also occurs during the molding and after the resin coating. Even if stress is applied, it is possible to prevent the leakage current from increasing. Furthermore, since the surface or inside is made of an aluminum alloy, it is flame retardant compared to pure niobium metal, and contributes to the improvement of the stability as a capacitor. Further, since the skeleton is niobium or niobium-aluminum alloy, the change in electric resistance value is small.

However, the alloying of niobium and aluminum has a large difference in melting point and specific gravity, so it is difficult to obtain a uniform alloy product unless a special method such as a rapid cooling method is used. Further, as a method of alloying the niobium surface, a method of applying aluminum alkoxide or the like after coating and internally diffusing by heating (Special Table 2003-514378) has been proposed. This method was made after hydrolysis of the alkoxide. There is a drawback that a complicated process such as reduction of aluminum oxide on the surface with a getter metal is required. In addition, oxygen diffused in the oxide is difficult to be reduced, and the conductivity of niobium is likely to be reduced, so that the equivalent series resistance (ESR) is likely to increase as a capacitor. Also, this oxygen tends to cause defects in the dielectric oxide film during chemical conversion.
Special table 2003-514378 gazette

  The present invention provides a method for inexpensively manufacturing a means for improving the thermal stability of a dielectric film made of niobium oxide, reducing the leakage current, and solving the problem of the surface hardness of the sintered body. Is. Moreover, the manufacturing method is given without reducing the electrical conductivity of the sintered body.

The solid electrolytic capacitor of the present invention, in order to solve the above problems, a niobium sintered body, and distributed in the aluminum diffusing agent containing aluminum powder, by heating, the aluminum niobium sintered body A method for producing a sintered body for a solid electrolytic capacitor to be diffused is provided.

According to the manufacturing method of the present invention, the following effects can be obtained.
Mixing and heating aluminum powder in niobium powder to diffuse aluminum in niobium powder, mixing aluminum diffusing agent in niobium powder and heating to diffuse aluminum in niobium powder, or niobium The sintered body is dispersed in an aluminum diffusing agent and heated to diffuse aluminum in the niobium sintered body, thereby improving the withstand voltage and reducing leakage current. Niobium powder capacitor materials can be supplied stably at low cost. Further, since there is no oxidation process as in the alkoxide method, it can be produced without causing a decrease in the conductivity of the sintered body.

Hereinafter, an embodiment of a method for producing a solid electrolytic capacitor powder or sintered body according to the present invention will be described.
The niobium powder used in the present invention is a crushed powder, a spherical powder, or a porous powder having an average particle diameter of about 1 μm to 200 μm, and preferably a porous powder having an average particle diameter of 10 μm to 100 μm. The niobium powder is not limited to pure niobium metal, but may be a niobium powder containing a small amount of boron, magnesium, silicon, phosphorus, titanium, copper, zirconium, molybdenum, hafnium, tantalum, tungsten, or the like, or niobium nitride. The aluminum powder used in the present invention is about 1 μm to 200 μm, preferably has an average particle size of about 5 μm to 100 μm, and preferably has a purity of 4N or more.

First, the following method is mentioned as a manufacturing method in which aluminum powder is mixed with niobium powder and heated to diffuse aluminum into niobium powder. Niobium powder and aluminum powder are mixed in a weight ratio of 30: 1 to 1: 2. A V-type mixer or the like can be used for mixing. After mixing for 10 minutes or more, the mixed powder is transferred to an airtight container, and argon gas is circulated from the lower part of the container. After replacing the argon gas for several minutes, it is set in an electric furnace, and the temperature is raised while flowing argon gas. The flowing gas is preferably an inert gas, and argon gas is particularly preferable. It is even better if high vacuum is used as the heating atmosphere. By heat treatment from 400 ° C. to 900 ° C., aluminum diffuses and penetrates into the niobium powder. Since an oxide film is formed on the surface of the aluminum powder, heating at 550 ° C. or higher is desirable. Heating is preferably 1 to 4 hours. While flowing argon gas, cool to near room temperature and take out the powder. The powder is treated with an acid other than hydrofluoric acid or an alkali to separate the niobium powder obtained by diffusing aluminum from the aluminum powder. Separation by specific gravity difference is also possible.
Next, as a manufacturing method for diffusing aluminum in niobium powder by mixing an aluminum diffusing agent containing aluminum powder with niobium powder and heating, the following method may be mentioned. Niobium powder is mixed with aluminum powder, ammonium halide powder and sintering inhibitor (alumina powder, aluminum nitride powder, etc.) as an aluminum diffusing agent and heated. Aluminum halide powder may also be mixed. As the halogen, chlorine, bromine, iodine, astatine or a mixture thereof is used, and particularly, an inexpensive ammonium chloride powder is used. Although it is possible to supply aluminum chloride as it is, since it is not easy to handle, it is possible to indirectly generate aluminum chloride by the following reaction formula and perform diffusion treatment. (S means solid, g means gas, x means number)
Al (s) + 3NH ₄ Cl (s) = AlCl ₃ (g) + 3NH ₃ (g) + 3 / 2H ₂ (g) ------------ (1)
3Nb (s) + AlCl ₃ (g) + 3 / 2H ₂ (g) = Nb ₃ Al (s) + 3HCl (g) ------------- (2)
Ammonium chloride (NH ₄ Cl) dissociates into hydrogen chloride (HCl) and ammonia gas (NH ₃ ) at 338 ° C or higher, and the generated HCl reacts with Al to react with aluminum chloride gas (AlClx) and hydrogen (H ₂ ) And generate. After AlClx is adsorbed on the surface of the Nb particles, chloride (Clx) is extracted by H ₂ and / or NH ₃ to form an active Al layer on the surface of the Nb metal. The Al layer diffuses into the Nb particles by heat treatment at 400 ° C or higher in an inert atmosphere. Heating is preferably 1 to 4 hours. While flowing argon gas, cool to near room temperature and take out the powder. The powder obtained by diffusion treatment of aluminum is obtained by separating the specific gravity difference. Next, the raw material powder produced by the above two methods embeds one end of the lead wire, forms pellets of a predetermined size by a dry press or slurry molding method, and vacuum-sinters to obtain a sintered body for a capacitor.
Next, the niobium sintered body manufactured normally is dispersed in an aluminum diffusing agent containing aluminum powder, and heated to diffuse aluminum in the niobium sintered body. The powder is formed into pellets of a predetermined size by a dry press or slurry molding method, and vacuum-sintered. The lead wire may be embedded before molding the pellet, or may be connected by welding or the like after aluminum is diffused. The sintered body is filled and arranged in the aluminum diffusing agent, heated while supplying argon gas, heated and held at a temperature of 400 ° C. to 700 ° C., and cooled. Next, the sintered body subjected to the diffusion treatment is separated from the diffusing agent by using a separating device such as a vibration sieve to obtain a sintered body for a capacitor.

  The sintered body produced by diffusing and infiltrating aluminum into the niobium surface prepared by the method of the present invention is then used in a chemical conversion solution such as phosphoric acid or nitric acid having a concentration of about 0.01 to 5 mol / l, for example, at 30 to 90 ° C. After boosting to a predetermined voltage at a current density of ˜40 mA / g, low voltage treatment for 1 to 3 hours is performed to form a dielectric film. Furthermore, a conductive polymer such as polyaniline, polypyrrole or polyethylenedioxythiophene, or a cathode layer such as a manganese dioxide layer is formed on the dielectric layer by a known method, and a carbon particle layer and a silver paste layer are anodic sintered. The electrodes are sequentially formed outside the body, and then the cathode terminal is connected thereto with a conductive paste or the like. After the anode lead wire is joined to the anode terminal by a technique such as welding, the resin sheath 8 is applied by a technique such as transfer molding to form a capacitor.

  Hereinafter, the present invention will be described in detail by way of examples.

2% by weight of camphor (camphor) was added to and mixed with a niobium porous powder having an average particle size of 70 μm to obtain a powder for press molding. A powder weight of 61 mg was collected and press-molded with a length × width × height of 4 mm × 3 mm × 2 mm. The molding density was 2.80 g / cm ³ . The molded body was subjected to a heating step such as debinding, and then vacuum sintered at 1410 ° C. for 20 minutes and cooled to obtain a niobium sintered body. The density of the sintered body was 3.13 g / cm ³ . The shrinkage rate is about 12%. It is also possible to add and mix an acrylic resin to the granulated powder to obtain a powder for press molding. The niobium sintered body is dispersed in an aluminum diffusing agent (Al: NH4Cl: Al2O3 = 1: 0.1: 2.9 (weight ratio)), and heated to 650 ° C. while flowing argon gas at 0.5 l / min. After holding at 2 ° C. for 2 hours, it was cooled to room temperature, and the sintered body was taken out using a vibration sieve. The aluminum concentration on the sintered body surface was 2.4% by weight. The buckling strength of the sintered body was improved by 1.6 times.
This sintered body was subjected to constant current formation at a current density of 20 mA / g in a 0.1 mol / l, 50 ° C. phosphoric acid aqueous solution, and then a constant voltage formation at 60 V for 2 hours, thereby combining niobium pentoxide and alumina. A chemical compound having a dielectric film was prepared. The film thickness of the dielectric was 192 nm. The dielectric coating is a composite oxide of niobium and aluminum or an oxide in which aluminum oxide is nano-dispersed.
On the dielectric film, a solid electrolyte layer was prepared by a conventional method as a counter electrode for the anode. A manganese dioxide layer was formed as the solid electrolyte layer. Further, a cathode layer made of graphite paste and silver paste was formed as in the conventional method. A metal terminal is joined to the cathode layer-forming element by welding and bonding for mounting using a conventional method, and a resin exterior is applied by transfer molding to improve moisture resistance and handling performance, resulting in a solid electrolytic capacitor It was. The leakage current was 0.2μA at a rated voltage of 6V applied for 1 minute and showed good performance. Even when the heat cycle from −50 ° C. to 125 ° C. was repeated 500 times, the ESR, leakage current, and capacity did not change. There was no change in capacity.

Claims (1)

A method for producing a sintered body for a solid electrolytic capacitor in which a niobium sintered body is dispersed in an aluminum diffusing agent containing aluminum powder and heated to diffuse aluminum into the niobium sintered body.